Because of excellent magnetic properties, rare earth sintered magnets as typified by Nd magnets are now widely used in motors, sensors and other components utilized in hard disk drives, air conditioners, hybrid vehicles and the like.
In general, rare earth sintered magnets are manufactured by the powder metallurgy via the following steps. First, raw materials are blended in accordance with a predetermined composition, melted in an induction melting furnace or the like, and cast into an alloy ingot. The alloy ingot is coarsely crushed by a grinding machine such as a jaw crasher, brown mill or pin mill, or by the hydrogen decrepitation process, and then finely ground by a jet mill or the like into a fine powder with an average particle size of 1 to 10 μm. The powder is pressed into a compact of desired shape in a magnetic field for imparting magnetic anisotropy, followed by sintering and heat treatment.
The in-magnetic-field pressing process involved in the manufacture of rare earth sintered magnets by the general powder metallurgy is a die pressing process comprising the steps of using a mold composed of a die, an upper punch and a lower punch, filling a cavity defined between the die and the lower punch with fine powder, and uniaxially pressing the powder between the upper and lower punches. It is a common practice to apply a lubricant to the interior surface of the die for reducing the friction between the upper and lower punches and the die interior surface and facilitating the release of the compact.
For the lubricant application, the method of spraying the lubricant to the interior surface of the die is generally employed. With this method, the molding operation is interrupted at every molding step or after a predetermined number of molding cycles, to take a time for lubricant applying operation. This means that the lubricant applying operation causes a lowering of productivity. It would be desirable to have a measure capable of efficiently applying the lubricant for thereby improving the productivity of rare earth sintered magnets.